The sense of feel is not typically used as a man-machine communication channel, however, it is as acute and in some instances as important as the senses of sight and sound, and can be intuitively interpreted. Tactile stimuli provide a silent and invisible, yet reliable and easily interpreted communication channel, using the human's sense of touch. Information can be transferred in various ways including force, pressure and frequency dependent mechanical stimulus. Broadly, this field is also known as haptics.
A single vibrotactile transducer can be used for a simple application such as an alert. Many human interface devices, for example a computer interface device, allow some form of haptic feedback to the user. A plurality of vibrotactile transducers can be used to provide more detailed information, such as spatial orientation of the person relative to some external reference. Using an intuitive body-referenced organization of vibrotactile stimuli, information can be communicated to a user. Such vibrotactile displays have been shown to reduce perceived workload by its ease in interpretation and intuitive nature (see for example: Rupert A H, 2000, Tactile Situation Awareness System: Proprioceptive Prostheses for Sensory Deficiencies. Aviation, Space, and Environmental Medicine, Vol. 71 (9):II, p. A92-A99).
The present invention relates to a low cost actuator assembly that conveys a strong, localized vibrotactile sensation (stimulus) to the body. These devices should be small, lightweight, efficient, electrically and mechanically safe and reliable in harsh environments, and drive circuitry should be compatible with standard communication protocols to allow simple interfacing with various avionics and other systems.
The study of mechanical and/or vibrational stimuli on the human skin has been ongoing for many years. Schumacher et al. U.S. Pat. No. 5,195,532 describes a diagnostic device for producing and monitoring mechanical stimulation against the skin using a moving mass contactor termed a “tappet” (plunger mechanical stimulator). A bearing and shaft is used to link and guide the tappet to the skin and means is provided for linear drive by an electromagnetic motor circuit, similar to that used in a moving-coil loudspeaker. The housing of the device is large and mounted to a rigid stand and support, and only the tappet makes contact with the skin.
The reaction force from the motion of the tappet is applied to a massive object such as the housing and the mounting arrangement. Although this device does have the potential to measure a human subject's reaction to vibratory stimulus on the skin, and control the velocity, displacement and extension of the tappet by measurement of acceleration, the device was developed for laboratory experiments and was not intended to provide information to a user by means of vibrational stimuli nor be implemented as a wearable device.
Various other types of vibrotactile transducers, suitable for providing a tactile stimulus to the body of a user, have been produced in the past. Prior vibrotactile transducers designs have incorporated electromagnetic devices based on a voice coil (loudspeaker or shaker) design, an electrical solenoid design, or a simple variable reluctance design. The most common approach is the use of a small motor with an eccentric mass rotating on the shaft, such as is used in pagers and cellular phones. A common shortcoming of these previous design approaches is that the transducers are rapidly damped when operated against the body—this is usually due to the mass loading of the skin or the transducer mounting arrangement (for example the foam material that would surround a vibrotactile transducer if it were mounted in a seat).
Pager motors, or eccentric mass EM motors, are usually constructed with a DC motor with an eccentric mass load such as half-circular cylinder that is mounted onto the motor's shaft. The motor is designed to rotate the shaft and its off-center (eccentric) mass load at various speeds. From the conservation of angular momentum, the eccentric mass imparts momentum to the motor shaft and consequently the motor housing. The angular momentum imparted to the motor housing will depend on the mounting of the motor housing, the total mass of the motor, the mass of the eccentric rotating mass, the radius of the center of mass from the shaft and the rotational velocity. In steady state, the angular momentum imparted to the housing will result in three dimensional motion and a complex orbit that will depend on the length of the motor, the mounting geometry, the length of the shaft and center of gravity of the moving masses (see for example J. L. Meriam, Engineering Mechanics: Dynamics, SI Version, 5th Edition, 2003, Wiley). This implementation applies forces in a continually changing direction confined to a plane of rotation of the mass. Thus the resultant motion of the motor housing is three dimensional and complex. If this motion is translated to an adjacent body, we may interpret the complex vibration (and perceived vibrational stimulus) to be diffuse and a “wobble” sensation.
The rpm of the EM motor defines the tactile frequency stimulus and is typically in the range of 60-150 Hz. Typically these devices are intended to operate at a single (relatively low) frequency, and cannot be optimized for operating over the frequency range where the skin of the human body is most sensitive to vibrational stimuli (see for example Verrillo R. T. (1992) “Vibration Sensation in Humans”, Music Perception, Vol 9, No 3, pp 281-302). It may be possible to increase the vibrational frequency on some EM motors by increasing the speed of the motor (for example by increasing the applied voltage to a DC motor). However, there are practical limits to this as the force imparted to the bearing increases with rotational velocity and the motor windings are designed to support a maximum current. It should also be apparent that the angular momentum and therefore the eccentric motor vibrational output also increases with rotational velocity which limits use of the device over bandwidth.
The temporal resolution of EM motors is limited by the start up (spin-up) times which can be relatively long, on the order of 100 ms or so. This is somewhat longer than the skin's temporal resolution, thus can limit data rates. If the vibrotactile feedback is combined with other sensory feedback such as visual or audio, the start-up delay has the potential of introducing disorientation. The slow response time needed to achieve a desired rotational velocity is due the acceleration and deceleration of the spinning mass—some motor control methods can address this by increasing the initial torque on turn on. It should be evident that motors with smaller eccentric masses may be easier to drive (and reduce spin-up time) however, thus far a reduced eccentric mass also results in an actuator that produces a lower vibrational amplitude.
There are two important effects associated with the practical operation of EM motors as vibrotacile transducers. Firstly the motion that is translated to an adjacent body will depend on the loading on the motor housing—from the conservation of momentum, the greater the mass loading on the motor (or transducer housing) the lower the vibrational velocity and perceived amplitude stimulus. Secondly, from the conservation of momentum, if the mass loading on the motor is changed, the torque on the motor and angular rotation rate will also change. In fact it is not possible to simultaneously and independently control output vibration level and frequency. This is obviously undesirable from a control standpoint, and in the limiting case, a highly loaded transducer would produce minimal displacement output and thus be ineffective as a tactile stimulus. In fact there have been several reports of inconsistency in results (Robert W. Lindeman, John L. Sibert, Corinna E. Lathan, Jack M. Vice, The Design and Deployment of a Wearable Vibrotactile Feedback System, Proceedings of the Eighth International Symposium on Wearable Computers (ISWC'04)) and modeling attempts to overcome this using complex mounting (Haruo Noma et al. A Study of Mounting Methods for Tactors Using an Elastic Polymer, Symposium on Haptic Interfaces for Virual Environment and Teleoperator Systems 2006). Thus depending on the mounting configuration, the displacement into skin and perception of vibrational stimulus is variable in frequency and level. This is obviously undesirable from a control standpoint, and in the limit, a highly loaded transducer would also produce minimal displacement output and thus be ineffective as a tactile stimulus.
Shahoian U.S. Pat. No. 6,697,043 B1 describes a computer mouse haptic interface and transducer that uses a motor transducer. This patent teaches the use a mechanical flexure system to convert rotary force from the motor to allow a portion of the housing flexure to be linearly moved. This approach relies on a complex mechanical linkage that is both expensive to implement and at high rotational velocities prone to deleterious effects of friction. It is therefore only suited to very low frequency haptic feedback.
In prior art, Shahoian U.S. Pat. No. 6,680,729 B1 an EM motor that is connected to the housing via a compliant spring. The system makes up a two degree of freedom resonant mechanical system. The motor mass and spring systems are completely contained within a rigid housing. The movement of the motor mass in this case acts to impart an inertial force to the housing. This type of transducer configuration is known as a “shaker”. The design claims improved efficiency and the ability to be driven by a harmonic motor drive for use as a haptic force feedback computer interface. The invention does not address any loading on the housing and in fact assumes that there are no other masses or mechanical impedances acting on the exterior of the housing.
Linear “shaker” transducers are well known in prior art, for example Clamme in U.S. Pat. No. 5,973,422 describes a low frequency vibrator with a reciprocating piston mass within a low friction bearing, actuated by an electromagnetic with a magnetic spring, having a spring constant K. The ratio of K to the mass M of the reciprocating member is made to be resonant in the operating frequency range of the vibrator. Other examples of prior art “shaker” transducer designs include U.S. Pat. Nos. 3,178,512, 3,582,875 and 4,675,907.
In summary, EM motors when used as vibrotactile transducers, provide a mounting dependent vibration stimulus and a diffuse type sensation, so that the exact location of the stimulus on the body may be difficult to discern; as such, they might be adequate to provide a simple alert such as to indicate an incoming call on a cellular phone, but would not be adequate to reliably provide spatial information by means of the user detecting stimuli from various sites on the body. The prior art fails to recognize the design requirements to achieve a small, wearable vibrotactile device that provides strong, efficient vibration performance (displacement, frequency, force) when mounted against the skin load of a human. This is particularly true when considering the requirement to be effective as a lightweight, wearable tactile display (e.g., multiple vibrotactile devices arranged on the body) in a high noise/vibration environment as may be found, for example, in a military helicopter. Further, the effect of damping on the transducer vibratory output due to the additional mechanical impedance coupled to the mounting has not been adequately addressed. The prior art further fails to effectively utilize an eccentric mass motor as the force generator in vibrotactile transducers or provide methods that extend the high frequency bandwidth and control the response of the transducer.
The foregoing patents reflect the current state of the art of which the present inventor is aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.